With various extreme climate phenomena occurring around the world, the threat of global climate changes caused by greenhouse gases such as CO2 is growing with each passing day, and it has reached an agreement to urge the reduction in greenhouse gas emission together. It is known that CO2 accounts for 77% of the greenhouse gas emission generated by human activities, and half of the total greenhouse gas emission is attributed to CO2 emission resulting from combustion of fossil fuels, thus, the reduction in CO2 emission is a critical point for realizing the reduction in greenhouse gases' emission. Common CO2 containing gas mixtures include flue gas, refinery gas, natural gas, synthesize gas, converted gas, and hydrogen-producing gas and the like. The concentration of CO2 in these gas mixtures ranging from 5% to 50%, and the other corresponding gas components including N2, O2, CO, H2, CH4, C2H6, SO2, H2S, as well as organosulfur compounds such as CH3S, COS, and so on.
Well-established methods of trapping and separating CO2 by absorption in domestic and international industries and the major industrially chemical absrobents include monoethanolamine (MEA), diethanolamine (DEA), di-isopropanolamine (DIPA), and methyl diethanolamine (MDEA), and the like. Furthermore, some sterically hindered amines such as 2-amino-2-methyl-1-propanol (AMP) may also be used as an absorbent because of its high CO2 absorption capacity. Among these methods, aqueous MEA solution is the most widely used ones contributing from its relatively high CO2 absorption efficiency and capacity. However, the aqueous MEA solution also has some drawbacks such as the tremendous amount of thermal energy required for the regeneration of solution as well as operational problems caused by chemical corrosion and degradation. In order to avoid excessive corrosion, generally only 10 to 30 wt % of MEA is contained in the aqueous MEA solution, with the rest is water. Since the solution containing 70% to 90% of water has to be heated to regenerate the aqueous MEA solution, as a result the uptake of water into the gas stream causes intensive energy consumption. Furthermore, other alkanolamine systems also show all sorts of drawbacks. To improve the CO2 absorption rate in some chemical absorbents such as di-isopropanolamine (DIPA) and methyl diethanolamine (MDEA) which usually show low CO2 absorption rate, monoethanolamine (MEA) and piperazine (PZ) are added, generating some so called improved absorption separation methods. However, drawbacks of corrosion and chemical degradation are still inherently connected to these mixing absorbents.
Adsorption separation is another CO2 capture method and has realized in industrial application in some places, while this kind separation method exists some drawbacks as well. As an example for the separation of gas mixtures in the conventional fixed bed adsorption separation tower, to realize a continuous separation process when using both pressure swing adsorption and temperature swing adsorption methods, a switching operation between the adsorption separation tower and regeneration tower should be conducted. At the same time, the big amount of adsorption heat released during the separation process abruptly increases the temperature of the bed layer, leading to a low separation efficiency. When using a moving bed or a simulated moving bed apparatus to perform continuous up-flow adsorption separation process, the adsorption capacity of the adsorbents is effectively utilized, but due to the complicated design of the up-flow procedure and low operation flexibility, therefore, such a method is only suitable to separate substances which have low selectivity and slow mass transfer rate.
In sum, it is important to improve the separation efficiency of conventional CO2 separation technologies or to develop more efficient and economic CO2 capture methods.